The catecholamine dopamine plays a major role in the regulation of cognitive, emotional and behavioral functions, and abnormalities in its regulation have been implicated in a number of psychiatric and neurological disorders. Dopamine acts through D2-like (D2, D3, D4) and D1-like (D1, D5) receptors, which are members of the seven transmembrane segment G protein-coupled receptor (GPCR) superfamily. The long-term goals of this project are: 1) to understand the structural bases of agonist and antagonist binding and specificity in dopamine receptors and related biogenic amine receptors and 2) to determine how agonist binding is transduced into G protein activation. Many drugs used to treat psychiatric disorders, including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and depression, target dopamine receptors, either directly or indirectly. We propose the following specific aims: 1. To determine the functional stoichiometry of the signaling unit of the dopamine D2 receptor (D2R), a representative Family A GPCR. We will determine whether two receptors signal through a single heterotrimeric G protein, the number of agonists required to activate a single signaling unit of D2R, and the role of agonist or antagonist binding to the second protomer in homomeric and heteromeric signaling units. 2. To determine the specificity of D2R heterodimerization in neurons in primary culture and in vivo in Drosophila melanogaster using biochemical, biophysical, and optical methods. 3. To differentiate the role of signaling of D2R homomers and heteromers using a novel resonance energy transfer based-biosensor for G protein activation. To complete our objectives we will combine experimental, structural, and computational approaches to receptor structure and function. In addition to work in heterologous cells, we will use mouse nucleus accumbens medium spiny neurons in primary culture and Drosophila melanogaster as a model system to dissect further the importance of homo- and heteromeric receptor signaling in vivo. Moreover, we will use our newly developed energy transfer based-biosensor to link biophysically the heteromeric receptor complex with G protein activation in real time. 7. PUBLICE HEALTH RELEVANCE: Many drugs used to treat psychiatric disorders, including schizophrenia, attention-deficit hyperactivity disorder (ADHD), and depression, target dopamine receptors, either directly or indirectly. That dopamine receptors may exist and function in homo- and heteromeric complexes with other GPCRs opens new pharmacological possibilities that will be best exploited if based on a clear understanding of the mechanistic basis of this signaling crosstalk.